Detecting the health of an inkjet nozzle has been a long standing problem in the field. In the case of scanning printheads, the ability to perform multiple passes has been used to minimize the impact of missing or improperly performing nozzles. As inkjet technology pushes into the laser printer performance space, printheads with nozzles spanning the entire page width have become more common. Using this printing method yields improved print speeds but no longer allows for multi-pass printing. Therefore, a method to verify that a nozzle is jetting properly is needed.
One such method is by optical detection as disclosed in U.S. Pat. Nos. 8,177,318, 8,376,506 and 8,449,068, as well as others. This method requires external light sources and sensors which can add cost and complexity to the printing device.
In an effort to eliminate the need for external devices, other methods have been disclosed which place impedance sensors on the ejector chip itself. One possible implementation of this method is described in U.S. Pat. Nos. 8,870,322, 8,899,709 and US Patent Application Publication No. 2014/0333694. These patents and applications teach the use of either differential or single ended impedance measurements taken over time to detect the formation and collapse of thermal vapor bubbles. It is further taught that different types of nozzle conditions such as blocked or weak nozzles can be determined by external processing of the data collected from the sensors. As disclosed particularly in U.S. Pat. No. 8,870,322, a method of calibration may be required to provide adequate performance of the system.
All of the prior attempts at determining condition of a printhead are based on detecting the formation of a bubble in an ink chamber. One shortcoming of this method is that by the time the bubble has reached the sensors the ejection event has passed. In most cases the detection of the bubble in the throat and chamber will occur 5 μs or more after the drop has been ejected.